The present invention is directed to a thyristor having forward and reverse blocking capability, composed of a p-emitter provided with an electrode forming an anode side, of an n-base, of a p-base and of an n-emitter provided with an electrode forming a cathode side. The p-base is composed of a p-conductive base region that is inserted into an n-conductive semiconductor body at an upper side thereof. The p-emitter is composed of a p-conductive semiconductor layer inserted into the semiconductor body at an under side thereof. The n-emitter is formed of at least one n-conductive region inserted into the p-base region. The present invention is also directed to the manufacture of such a thyristor.
A thyristor of this type is disclosed in the text Thyristor-Handbuch by A. Hoffmann and K. Stocker, published by Siemens AG, Berlin and Munich, 1965, pp. 37 and 38, particularly FIG. 8.1. Also as described in the book by P. D. Taylor, Thyristor Design and Realization, John Wiley and Sons, New York, 1987, pp. 28-35, particularly FIG. 2.4, a pn-junction between the n-base and the p-base is biased in a non-conducting direction when a reverse voltage is applied across the anode and cathode of a thyristor that places the anode at a higher potential than the cathode. By contrast when a reverse voltage is applied across the anode and cathode as a result whereof the cathode is placed at a higher potential than the anode, the pn-junction between the n-base and the p-emitter is biased in a non-conducting direction. When the reverse voltage that is applied exceeds a prescribed limit value that defines the positive blocking capability (forward blocking capability) of the thyristor, t hen a breakdown occurs at the pn-junction between the n-base and the p-base, the result being that the thyristor undesirably triggers and may be highly thermically loaded under certain circumstances. When, on the other hand, the reverse voltage that is applied exceeds a prescribed limit value that defines the negative blocking capability (reverse blocking capability) of the thyristor, then an avalange breakdown of the pn-junction between the n-base and the p-emitter occurs that generally thermically overloads and thus destroys the thyristor.
At those locations at which the aforementioned pn junctions reach the semiconductor surface, the electrical field strength increases due to the influence of surface charges and edge geometry. As a result, a premature breakdown occurs at the component edge before the level of blocking capability is reached. There have been attempts to reduce the surface field strengths appearing at the edges of the pn-junctions that are biased in the non-conducting direction.
For increasing the surface-side breakdown voltage of a planar pn-junction that separates a semiconductor region driven into the semiconductor body from the remaining part of the semi-conductor body, European Patent A 0 176 778 discloses the use of a diffusion mask that is provided with smaller, additional openings outside of an opening that laterally defines the semiconductor region. As a result a doping profile derive from drive-in of the dopant that very gradually approaches the boundary surface of the semiconductor body with increasing distance from the edge of the mask opening that defines the semiconductor region.